Method for nucleation and deposition of diamond using hot-filament DC plasma

ABSTRACT

A method and apparatus for nucleation and growth of diamond by hot-filament DC plasma deposition. The apparatus uses a resistively heated filament array for dissociating hydrogen in the reactant gas. For two sided diamond growth, configurations of substrate-hot filament-grid-hot filament-substrate or substrate-hot filament-hot filament-substrate configuration are used. For the latter configuration, two independent arrays of filaments serve as both hot filament and grid, and AC or DC plasma is maintained between the filament arrays. For this and the other electrode configurations, the grid electrode is positively biased with respect to the hot filaments to maintain a plasma. The plasma potential gradient across the grid and the hot-filament draws ions from the plasma towards the filaments. To further increase deposition rates, the filament array is biased negatively with respect to the substrate holder so that a DC plasma is also maintained between the substrate and filament array. During nucleation, the filament adjacent to the substrate holder is biased positively relative to the substrate so that more ions are accelerated towards the substrate, which in turn enhances the flow of growth precursors towards the substrate resulting in a high diamond nucleation density on the substrate without the need for scratching or diamond-seeding pretreatment. This nucleation method simplifies the growth process and provides a convenient and economical means for heteroepitaxial growth of diamond nuclei on single crystal substrates like Si (100).

CROSS-REFERENCE TO RELATED US PATENT APPLICATION

This patent application is a divisional patent application of U.S.patent application Ser. No. 08/888,830 filed on Jul. 7, 1997, entitledAPPARATUS AND METHOD FOR NUCLEATION AND DEPOSITION OF DIAMOND USINGHOT-FILAMENT DC PLASMA, allowed and which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of chemical vapor deposition(CVD) of diamond films, and more particularly, to a method and apparatusfor nucleation and growth of diamond films by hot filament DC plasmaCVD.

BACKGROUND OF THE INVENTION

Hot filament chemical vapor deposition (HFCVD) has been extensively usedby researchers to deposit polycrystalline diamond on a variety ofsubstrates. The technique and reactor designs typically used for HFCVDof diamond are described in detail in an article entitled “Growth OfDiamond Particles From Methane-Hydrogen Gas” published in J. MaterialsScience 17, 3106(1982) by Matusumoto et al. Since this disclosure,numerous researchers have attempted to improve the HFCVD technique. Thisdevelopment can be found in the review article by C. E. Spear entitled“Diamond-ceramic coating of the future” published in J. Am. Ceram. Soc.72(2), 171(1989). The reactor generally comprises a resistively heatedfilament and a heated or cooled substrate stage which are housed in areactor chamber with pumping and pressure monitoring facilities. Thefilament is made from a high melting-point refractory metal that is usedto dissociate hydrogen and other molecules in a feed gas which normallycontains a mixture of hydrogen and hydrocarbon. Atomic hydrogen andother dissociated products subsequently react with the feed gas togenerate precursors responsible for diamond formation. The precursorsthen diffuse to and condense on the substrate for the formation ofpolycrystalline diamond. The separation between the filament and thesubstrate is normally in the range 0.5 to 5 cm. With this smalldistance, a sufficient amount of growth precursors diffuses to thesubstrate prior to their recombination into more stable molecules.

A major advantage of HFCVD of diamond films, relative to other methodsof diamond film growth such as microwave plasma CVD (MWCVD),radiofrequency CVD, and plasma jet CVD, is the low equipment investmentcosts, and the ease in scaling up the production to a large areasubstrate. The diamond growth rate using HFCVD does not normally exceed5 μm/hr and is typically about 1 μm/hr (see e.g., International PCTPatent Publication WO 91/14798, by Garg, et al., entitled “An ImprovedHot Filament Chemical Vapor Deposition Reactor”), which is not highenough for economically viable thick film production. A majordisadvantage of HFCVD, as with other known diamond growth methods, isthat it requires scratching or diamond-seeding of the substrate surfaceto initiate diamond nucleation. Such a pretreatment induces a highdefect concentration on the substrate surface and thus generallyprecludes the possibility of obtaining heteroepitaxial growth ofdiamond. This pretreatment increases the CVD diamond production costs.

A method of achieving nucleation enhancement is disclosed by Yugo et al.in an article entitled “Generation Of Diamond Nuclei By Electric FieldIn Plasma Chemical Vapor Deposition” and published in Applied PhysicsLetters 58(10), 1036-1038(1991) which proposes a predeposition ofdiamond nuclei on a silicon mirror surface prior to the conventionaldiamond CVD growth process. Yugo et al. reported that diamond nucleigrowth required a high methane content in hydrogen and did not occurbelow 5%, and that high densities of nuclei occurred only above 10%methane. Yugo et al. also reported that the substrate bias against theCVD plasma should be below 200 volts to avoid sputtering and the typicalbias was 70 volts. The total time duration for the pretreatment waslimited to between 2 to 15 minutes.

More recently, Stoner et al. (see e.g., World Patent # 93/13242,entitled “Nucleation Enhancement For Chemical Vapor Deposition OfDiamond”) and Jiang et al. (see e.g., “Epitaxial Diamond Thin Films On(001) Silicon Substrates”, Applied Physics Letters 62(26),3438-3440(1993)) have independently disclosed diamond nucleationenhancement by negatively biasing the substrate against the CVD plasmaduring MWCVD of diamond films on silicon. More importantly, both ofthese groups showed that the preservation of the crystallinity of thesilicon substrate surface as a result of the elimination of anyscratching/diamond-seeding pretreatment, together with the nucleationenhancement, allows the heteroepitaxial formation of diamond (100)nuclei on Si (100). In the method described by Jiang, et al., thesubstrate was biased at −100 to −300 V relative to the microwave plasmawith a typical recipe for MWCVD of diamond using CH₄/H₂. In the methoddescribed by Stoner et al., the negative bias of the substrate requiredfor nucleation enhancement was claimed to be not less than 250 volts.The nucleation of diamond and heteroepitaxial nucleation of diamond witha modified HFCVD-DC plasma method and apparatus, which require much lessequipment investment than the MWCVD approach, is one advantage of thepresent invention discussed hereinafter.

Modifications of the conventional HFCVD by coupling it with DC plasmaCVD have been proposed previously by A. Ikegaya and T. Masaaki in JP173366 (1986), JP 75282 (1987), and European Patent Publication 0254312A1. In this approach, a hot filament array is used as a thermionicelectron emitter and a grid electrode is inserted between the hotfilament array and the substrate. The filament array and the substrateare both negatively biased against the grid electrode in order to formtwo DC plasma zones, one between the filament array and the gridelectrode, and the other between the grid electrode and substrate. Inthese two plasma zones, the plasma density in the grid-filament zone ismuch higher than that between the grid-substrate zone because ofthermionic electron emission from the hot filaments. In thegrid-filament zone, ions are extracted towards the filaments, i.e.,further away from the substrate. Through gas phase collision, thisextraction will also move the reactants generated near the filamentaway, instead of towards, the substrate. Ikegaya et al. reported agrowth rate of 2 μm/hr on a tungsten carbide substrate using 1% methanein hydrogen, a power density of 40 W/cm² between the hot filament andgrid and 20 W/cm² between the grid and substrate with a hot filamenttemperature of about 2000° C. and a substrate temperature of 980-1010°C. and a pressure of 90 torr. A growth rate of 12.5 μm/hr was alsoreported for a gas mixture of 2% (CH₃ ₂CN in H₂ with a power density of60 W/cm² between the filament and grid, and a power density of 40 W/cm²between the grid and substrate. Ikegaya et al. noted that a DC plasmapower density higher than 200 W/cm² between the grid and substrate ledto sputter-etching of the substrate. Ikegaya et al. reported that thisproblem arises because the negative bias on the substrate against thegrid attracts ions to the substrate. A high DC plasma power densityresults in a high bombardment energy and high current density, and theinduced energetic particle bombardment causes detrimentalsputter-etching.

A logical approach for eliminating the sputtering problem is to connectthe substrate to the grid or simply to discard the grid. A DC plasma canstill be maintained by biasing the filament negatively against thesubstrate. In fact, A. Ikegaya and N. Fujimori showed such aconfiguration in a JP 176762 and a PCT Patent Publication WO92/01828.However, a drawback to both of these designs is that they do not allowfor any ion extraction towards the substrate during nucleation andgrowth.

Thus, there still exists a need to modify the HFCVD method and apparatusin order to provide an economical approach to control energetic particlebombardment for improved diamond nucleation and growth.

SUMMARY OF THE INVENTION

The present invention discloses a method and apparatus to nucleatediamond in a high density on substrates without any scratching ordiamond-seeding requirement, and to efficiently grow diamond coatings athigh growth rates. The design of the method and apparatus takes intoaccount the limitations of the aforementioned hot filament DC plasmadevices and processes.

The present invention provides a method of growing a diamond film by hotfilament discharge comprising positioning a substrate having adeposition surface on a substrate holder in a vapor deposition chamber,providing a grid electrode spaced from the substrate deposition surfaceand providing a filament array electrode interposed between the gridelectrode and the substrate deposition surface. The method includesflowing a gas mixture comprising hydrogen and gas containing carbon intothe vapor deposition chamber and resistively heating the filament arrayelectrode to a temperature in a range of from about 1800° C. to about2600° C. with the substrate being heated to a temperature in the rangefrom about 600° C. to about 1100° C. The method includes nucleating thesubstrate by biasing the filament array electrode at a positive voltagewith respect to the substrate holder, and biasing the grid electrode ata voltage positive with respect to the filament array electrode.Thereafter the grid electrode is biased at a voltage positive withrespect to the voltage on the filament array electrode to grow a diamondfilm on the deposition surface.

During the step of nucleating the substrate the substrate may be biasedat ground potential and the filament array electrode biased to apotential in a range from about 20 to about 300 Volts with respect tothe substrate holder. The grid electrode is biased to a voltage in arange from about 20 to about 300 Volts with respect to the filamentarray electrode.

During the step of growing the diamond film after the step ofnucleating, the substrate holder and filament array electrode may bebiased at ground potential and the grid electrode biased at a voltage ina range from about 20 to 300 Volts with respect to the filament arrayelectrode.

Alternatively, during the step of growing the diamond film after thestep of nucleation the substrate holder may be biased at groundpotential and the filament array electrode biased at a negative voltagewith respect to the substrate holder wherein the negative voltage beingin a range from about −20 to about −300 Volts with respect to thesubstrate holder.

In another aspect of the invention there is provided a hot filament DCdischarge plasma apparatus for synthesizing a diamond film. Theapparatus comprises a deposition chamber having a gas inlet for flowingreactant gases into the deposition chamber; a conducting substrateholder having a surface for holding a substrate, and means for heatingand cooling the substrate holder. The apparatus includes a gridelectrode spaced from the surface of the substrate; a filament arrayelectrode interposed between the grid electrode and the surface of thesubstrate holder, and includes means for resistively heating thefilament array electrode. The apparatus includes means for biasing thegrid electrode, the filament array electrode and the substrate holder toproduce a hot filament DC discharge plasma. The means for biasingincludes means for adjusting the bias potential on the grid electrodeand the filament array electrode relative to the substrate holder andeach other.

In another aspect of the invention there is provided a hot filament DCdischarge plasma apparatus for synthesizing a diamond film, comprising adeposition chamber having a gas inlet for flowing reactant gases intothe deposition chamber; first and second spaced conducting substrateholders each adapted to support a substrate having a surface on whichthe diamond film is to be synthesized, and means for heating and coolingthe first and second substrate holders. The invention includes a gridelectrode located between the first and second substrate holders; afirst filament array electrode interposed between the first substrateholder and the grid electrode and a second filament array electrodeinterposed between the second substrate holder and the grid electrode.The apparatus includes means for resistively heating the first andsecond filament array electrodes. The apparatus is provided with powersupply means for biasing the grid electrode, the first and secondfilament array electrodes and the first and second substrate holders toproduce a hot filament DC discharge plasma and including means foradjusting the bias potential on the grid electrode, and on the first andsecond filament array electrodes relative to the first and secondsubstrate holders respectively.

In another aspect the present invention provides a hot filamentdischarge plasma apparatus for synthesizing a diamond film comprising adeposition chamber having a gas inlet for flowing reactant gases intothe deposition chamber; first and second spaced conducting substrateholders each adapted to support a substrate having a surface on which adiamond film is to be synthesized, and means for heating and coolingfirst and second substrate holders. The apparatus includes a firstfilament array electrode spaced from the first substrate holder and asecond filament array electrode interposed between the first filamentarray electrode and the second substrate holder, and means forresistively heating the first and second filament array electrodes. Theapparatus includes means for biasing the first and second filament arrayelectrodes and the first and second substrate holders to produce a hotfilament discharge plasma and includes means for adjusting the biaspotential on the first and second filament array electrodes relative tothe first and second substrate holders respectively.

More particularly, in the present invention a hot filament is placedbetween a grid electrode and the substrate. The plane of the grid isparallel to the plane of filament. The grid can be either parallelwires, or rods, or mesh, or plate with holes. The grid can be eithercooled or heated. The direction of the elements of the grid (wires orrods) can be either perpendicular or parallel, or at any angle to thedirection of filament. The distance between the hot filament andsubstrate is preferably less than 2 cm, and the distance between hotfilament and grid is preferably less than 5 cm. In the normal operationof the system, the growth substrate holder is preferably biased atground potential. The power density for resistively heating thefilaments is about 30-500 W/cm². During nucleation of diamond, thefilaments are biased positively at 20-300 Volts relative to thesubstrate holder and the grid electrode is biased positively in therange of 20-300 Volts relative to the filament array. As such, a DCplasma can be maintained between the grid and filaments. Ions in theplasma are extracted towards the substrate for particle bombardmentassisted nucleation. The nucleation process typically takes less thanten minutes. During diamond nucleation, a plasma is maintained betweenthe filaments and grid with the thermionic emission from the hotfilament cathode to enhance the plasma density. The unique substrate-hotfilament-grid configuration of the present invention allows themaintenance of the substrate at a potential even more negative than thehot filament cathode such that effective ion extraction towards thesubstrate can be induced for the enhanced diamond nucleation.

During diamond growth, the substrate holder may be biased at groundpotential. The filament array is either not biased at all or biasednegatively relative to the substrate holder. The negative biasingvoltage on the filament array is normally −20 to −300 Volts. The grid isbiased positively at 20-300 Volts relative to the filament array. Thetypical plasma energy density is about 20-300 W/cm². When the filamentsare not biased, a DC plasma will be mainly maintained between the gridand filaments. The large cathode voltage drop near the filament willdraw some ions from the plasma towards the filaments. Due to the smallmean free path for collision in the processing pressure (about 0.01 mmat 50 Torr and 1500K), such an ion extraction in the direction towardsthe substrate will result in the partitioning of the ion energy intokinetic energy for neutrals in the collision cascades. In turn, theseaccelerated neutrals will have a net average velocity towards thesubstrate, and have an average energy higher than the average thermalenergy in the system, and can thus enhance the reaction probabilitytowards diamond growth. But the average energy will be much less than afew electron volts, which is not sufficient for inducing any significantsputtering action on the substrate. The motion of neutrals towards thesubstrate enhances the arrival rate of the precursors beyond thatinduced by simple diffusion, and enhances the diamond growth rate.

With the filament array biased at a negative potential relative to thesubstrate holder, a DC plasma can also be maintained between thefilament array and substrate. However, a high DC power input in thisoperation mode may lead to the raise of the substrate surfacetemperature outside the diamond growth window, which is a limitationcommon to other prior arts of hot filament DC plasma techniques.Accordingly, under the normal operation in the present invention, theplasma power input between the grid and filaments is higher than thatbetween the filaments and substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The method and apparatus for growing diamond coatings using thehot-filament DC plasma method according to the present invention willnow be described, by way of example only, reference being had to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a conventional HFCVD reactor accordingto typical configurations of prior art;

FIG. 2 is a schematic diagram of a hot filament DC plasma CVD reactorwith a typical filament-grid-substrate configuration of prior art;

FIG. 3 is a schematic diagram of the apparatus according to the presentinvention;

FIG. 4 is a schematic diagram of an apparatus for depositing diamondcoatings onto multiple substrates according to the present invention;

FIG. 5 is a schematic diagram of another embodiment of an apparatus fordepositing diamond coatings;

FIG. 6a is an optical micrograph of heteroepitaxially grown diamond onSi (diamond (100)//Si(100), diamond [110]//Si[110]) produced by thepresent invention in a low nucleation density mode; and

FIG. 6b is an optical micrograph of heteroepitaxially grown diamond onSi (diamond (100)//Si(100), diamond [110]//Si[110]) produced by thepresent method in a high nucleation mode.

DETAILED DESCRIPTION OF THE INVENTION

Prior Art

A schematic representation of a typical HFCVD reactor used in prior artdiamond growth processes is shown in FIG. 1. The reactor 10 comprises achamber 12 enclosing a resistively heated filament 14 and a heated orcooled substrate holder 16 on which a substrate 17 is placed. Pumps andpressuring monitoring equipment is included (not shown). The reactantgas mixture is fed into the chamber through a gas diffuser unit 18. Thefilament 14 is made from a high melting-point refractory metal, such astungsten or tantalum, and is heated to between 1800-2300° C. todissociate hydrogen and other molecules in the reactant gas mixturewhich normally contains a mixture of hydrogen and hydrocarbon. Atomichydrogen and other dissociated products subsequently react with thereactant gas mixture to generate precursors responsible for diamondformation. The precursors then diffuse to and condense on substrate 17for the formation of polycrystalline diamond. The separation between thefilament and the substrate is normally in the range of 0.5 to 5 cm. Thetemperature of the substrate is generally maintained in the range of 700to 1000° C. The deposition rate and reaction efficiency are determinedby a combination of the rate of reactant generation near the filaments,the rate of reactant diffusion to the substrate, and the probability ofdiamond formation on the substrate.

FIG. 2 illustrates several modifications of the conventional HFCVD bycoupling the process with direct current (DC) plasma CVD as proposed byA. Ikegaya and T. Masaaki in JP 173366 (1986), JP 75282 (1987), andEuropean Patent 0254312 A1. Referring to FIG. 2, in this approach, a hotfilament 20 is used as a thermionic electron emitter and a gridelectrode 22 is inserted between the hot filament 20 and the substrate24. The filament 20 and the substrate 24 are both negatively biasedagainst the grid electrode 22 using power supplies 26 in order to form aDC plasma between the filament 20 and grid 22, and the grid 22 andsubstrate 24.

Present Invention

FIG. 3 is a schematic diagram of an apparatus 40 for implementing themethod of nucleating and growing diamond in accordance with the presentinvention. The reactant gas mixture is introduced into a depositionchamber 42 through a gas shower unit 44. The substrate 46 on which thediamond film is being deposited is placed on a substrate holder 48 whichmay be heated or cooled by a heat exchange fluid running throughconduits 50. The substrate holder 48 is equipped with a thermocouple 52for sensing the temperature of the substrate holder and may be connectedto a substrate temperature controller 54 that controls the temperatureof the heat exchange fluid.

A filament array 58 is mounted on conducting rods 60 and is spaced abovesubstrate holder 48. A grid electrode 64 is mounted on conducting rods66 and is spaced above filament array 58 so that a sequentialgrid-filament-substrate assembly is provided. The filament array 58 ispreferably spaced less than 2 cm from the top of substrate 46. The gridelectrode 64 is preferably spaced less than 5 cm from filament array 58.Grid electrode 64 is shown as a wire grid in FIG. 3 but may also beconstructed from a wire mesh, metal rods or a perforated metal plateable to withstand the operating temperatures.

Filament array 58 is comprised of a plurality of conductive metalfilaments with a high melting point such as Ta and W. Filament array 58can withstand resistive heating to temperatures above 2000° C., thepreferred temperature range for dissociating hydrogen in the reactantgas. Grid electrode 64 and filament array 58 are shown as beingsubstantially parallel in chamber 42 but it will be appreciated thatthese components do not need to be parallel to each other.

DC power supplies 72 and 74 are used to provide the DC bias requirementsduring substrate nucleation and diamond coating growth. Morespecifically, power supply 74 is used to maintain the bias requirementsbetween the filament array 58 and substrate 46 and power supply 72provides the bias requirements between the grid electrode 64 andfilament array 58. It will be understood that the substrate on which thediamond film is being deposited is typically electrically conductive sothe substrate will be biased to the same potential as the substrateholder. During diamond deposition the filament array 58 is maintained inthe temperature range of 1800-2600° C. using a power supply 78 which ispreferably an AC power supply. The power density is about 30-500 W/cm².The plasma power density during growth is about 20-300 W/cm². The gridelectrode 64 may be either heated or cooled during deposition of thediamond film. The grid electrode 64 may be heated resistively and/or byplasma energy. Grid 64 may comprise hollow rods and may be cooled byrunning a heat exchange fluid through the centre of the electrode rods.

The temperature of filament array 58 and grid electrode 64 is monitoredby an optical pyrometer (not shown) which is located outside thedeposition chamber 42 and focussed onto filament array 58 through awindow 70 in the vacuum chamber. The gas flow and pressure arecontrolled by conventional flow meters and controllers, vacuum pumps andgauges (not shown).

The reactant gas mixture comprises hydrogen, at least one carbon sourceincluding hydrocarbons, hydrocarbons containing oxygen and/or nitrogen,hydrocarbons containing halogens, carbon vapor, CO, CO₂, and optionallyother gases such as O₂, F₂, and H₂O. The reactant gas pressure is set inthe range between 10 to 500 Torr. The power density is about 30-500W/cm². The plasma power density during growth is about 20-300 W/cm².

Referring to FIG. 3, in the normal operation of the system, the growthsubstrate 46 is maintained at ground potential for both the step ofnucleation and diamond growth. The power density for resistively heatingthe filament array 58 is about 30-500 W/cm². During the step ofnucleation of diamond film the filament array 58 is biased positivelywith respect to the substrate, preferably in the range of 20-300 Voltspositive with respect to the substrate. It will be understood thatmaintaining the substrate at ground potential is preferred but it couldbe maintained at potentials in the vicinity of ground so long as thefilament array 58 is at a more positive potential than the substrateholder. The grid electrode 64 is biased positively with respect to thefilament array, preferably in the range of 20-300 Volts with respect tothe filament array 58 so that during nucleation the grid electrode 64 ismaintained at a voltage more positive than the filament array.Therefore, during diamond nucleation, a plasma is maintained between thefilament electrode 58 and grid 64 and thermionic emission from theheated filament array 58 enhances the plasma density. Ions in the DCplasma are extracted towards the substrate 46 for particle bombardmentassisted nucleation. The nucleation process according to the methoddisclosed herein advantageously takes less than ten minutes. The uniqueconfiguration of the present invention allows the maintenance of thesubstrate 46 at potentials more negative than the heated filament array58 thereby resulting in ion extraction towards the substrate 46 toachieve enhanced diamond nucleation.

During the step of diamond growth, the filament array 58 is eitherelectrically connected to the substrate holder 48, or alternatively isbiased negatively relative to the substrate holder 48, preferably in therange of −20 to −300 Volts. Grid electrode 64 is biased positively withrespect to the filament array 58, and preferably in the range 20-300Volts with respect to the filament array 58 whether or not the filamentarray is held at ground. The typical plasma energy density is about20-300 W/cm². When the filament array 58 is not biased with respect tothe substrate holder 48 (so that both are at the same potential), a DCplasma will be maintained between the grid electrode 64 and filamentarray 58. The large cathode voltage drop near the filament array 58 willextract some ions from the plasma towards the filaments. Due to thesmall mean free path for collision at the processing pressure (about0.01 mm at 50 Torr and 1500K), such an ion extraction in the directiontowards the substrate 46 will result in the partitioning of the ionenergy into kinetic energy for neutrals in the collision cascades. Inturn, these accelerated neutrals will have a net average velocitytowards the substrate 46 and an average energy higher than the averagethermal energy in the system thereby resulting in an enhanced reactionprobability. However, the average energy will be much less than a fewelectron volts, which is not sufficient for inducing any significantsputtering action on the substrate 46. The motion of neutrals towardsthe substrate 46 enhances the arrival rate of the growth precursorsbeyond that induced by simple diffusion, thereby enhancing the diamondgrowth rate.

In the alternative case in which the filament array 58 is biased at anegative potential relative to the substrate holder 48, a DC plasma canalso be maintained between the filament array 58 and substrate 46.However, a high DC power input in this operation mode may lead to anincrease in temperature of the substrate surface outside the diamondgrowth window, which is a limitation common to other prior art hotfilament DC plasma growth techniques. Accordingly, under the normaloperation in the present invention, the plasma power input between thegrid electrode 64 and filament array 58 is higher than that between thefilament array and substrate 46.

FIG. 4 shows a substrate-hot filament-grid-hot filament-substrateconfiguration at 90 for depositing diamond coatings onto two substrates46′ affixed to the opposed substrate holders 48. The array 90 may beassembled in vacuum chamber 42 vertically as shown or alternatively theentire assembly may be rotated 90° to a horizontal position within thechamber. The two hot filament arrays 92 and 94 may be heated either byindependent DC or AC power supplies, or by a shared DC or AC powersupply (not shown). Each of the two filament arrays 92 and 94 areprovided with a DC power supply for biasing the arrays with respect toground (not shown). A grid electrode 96 is located between filamentarrays 92 and 94 and is biased using a DC power supply (not shown) andis biased at a positive potential relative to the filament arrays 92 and94, preferably in range from 20-300 Volts. During the nucleation step,the filament arrays 92 and 94 are biased at a positive potentialrelative to the associated substrates 46′ adjacent thereto. Duringdiamond growth following the nucleation step, the filament arrays 92 and94 are either not biased at all or biased negatively with respect theassociated substrates 46′, similar to the process described above withrespect to the apparatus of FIG. 3. The operation range is the same asthose discussed in relation to FIG. 3.

FIG. 5 shows another configuration 100 for growing diamond coatingsaccording to the present invention. Two filament arrays 102 and 104 areresistively heated by independent AC or DC power supplies (not shown).Both groups of hot filament arrays 102 and 104 serve the function ofgrid electrodes so that in operation the two filament arrays are biasedappropriately to maintain the plasma discharge between the two filamentarrays using either an AC or DC power supply, preferably an AC powersupply (not shown).

The following non-limiting examples are to further illustrate thepresent invention.

EXAMPLE 1

Nucleation On Mirror-Smooth Quartz

Diamond nucleation even on mirror-smooth quartz was achieved by biasingfilament array 58 at 89 Volts and heated to a temperature of about 2160°C. and biasing grid 64 at 200 Volts using the apparatus of FIG. 3. Thereactant gas mixture was a mixture of methane/H₂ and respective flowrates were 6.5 standard cubic centimeters per minute (sccm) for methaneand 300 sccm for hydrogen at a total pressure of 30 Torr. The nucleationprocess was maintained for about 10 minutes. The bias on filament array58 was then switched off and the bias on grid electrode 64 was adjustedto 120 Volts for diamond growth. A coherent and uniform well faceteddiamond film was obtained. Under the same growth conditions but withoutthe nucleation step, only patches of diamond with non-uniform thicknesswere formed on mirror-smooth quartz. Subsequent runs showed that thenucleation time could be in the range of 2-5 minutes.

EXAMPLE 2

Heteroepitaxy Of Diamond On Silicon (see FIG. 6)

Diamond oriented crystals were grown on silicon (100) by pre-cleaningthe silicon with an HF solution, and nucleating with a filamenttemperature of about 2200° C., a grid bias of 219 Volts, and a filamentbias of 130 Volts using the method and apparatus of FIG. 3. The reactantgas mixture was a mixture of methane/H₂ and respective flow rates were 6sccm for methane and 300 sccm for hydrogen at a total pressure of 50Torr. The nucleation time was about 10 minutes. The filament array wasthen biased to zero volts for diamond growth. The grid bias was changedto 112 Volts in this process. FIG. 6a clearly shows diamond (100) cubiccrystals with its (100) face aligned with the Si (100), and the diamonddirection [110] aligned with that of Si [110]. When diamond was grownwith a high nucleation density, a coherent film with diamond(100)//Si(100), and diamond [110]//Si[110] was formed, as that shown inFIG. 6b.

EXAMPLE 3

Fast Deposition Of Diamond Films

A diamond coating was grown for 160 hours to 2.5 mm in thickness and 2″in diameter using the apparatus of FIG. 3. The pressure for diamondgrowth was 30 Torr and bias voltage on the grid electrode was 45 Voltsrelative to the filament array. The filament power density was about 170W/cm² and the plasma power density was 40 W/cm². The growth rate was 16μm/hr. Both Raman and X-ray photoelectron spectroscopy showed purediamond and no impurities in the sample (data not shown).

EXAMPLE 4

Further Fast Deposition Of Diamond Films

A diamond coating was grown for 44 hours to 0.93 mm in thickness and 2″in diameter on a substrate using the apparatus of FIG. 3. The pressurefor diamond growth was 30 Torr and bias voltage on the grid electrodewas 50 Volts relative to the filament array. The growth rate was 21μm/hr. Both Raman and X-ray photoelectron spectroscopy showed purediamond and no impurities in the sample (data not shown). The filamentpower density was about 170 W/cm² and the plasma power density was about50 W/cm².

The present method is advantageous over the diamond film growth processdisclosed in EP0254560 because in the latter an ion extraction assisteddiamond nucleation step cannot be performed due to the fact that the hotfilaments are effective electron emitters, and thus most effectivelyused as a cathode in a DC plasma configuration. In EP0254560, thesubstrate can only be used effectively as an anode, and ions in theplasma are attracted to the cathode instead of anode.

The present method is advantageous over the diamond film growth processdisclosed in EP0254312 because the grid electrode in EP0254312 islocated between the filaments and substrate, and is always biasedpositively to the filaments to maintain a DC plasma. The ion extractionfrom the plasma between the grid and filaments towards the cathode hotfilaments will induce a net flow of neutrals in the ion-neutralcollision cascades away from the substrate. As such, many growthreactants generated on and near the hot filaments, and those generatedin the plasma are not advantageously utilized and therefore wasted.Although when the substrate is biased negatively to the grid, some ionsfrom the plasma can be extracted towards the substrate, these ions areextracted from the anode (the grid) of the DC plasma between thefilaments and the grid. Hence, the extraction is not efficient, ascompared to the technology disclosed in the present application.

Although a DC plasma can indeed be maintained by using the substrate asthe cathode and grid as the anode, a DC glow discharge from two parallelelectrodes separated by 1 cm in the typical diamond growth pressure of50 Torrs will require a DC voltage much higher than that from theconfiguration illustrated in FIG. 3 for the same ion current density onthe substrate. This is because the maintenance of a DC plasma betweentwo cold electrodes relies on secondary electron emission as aconsequence of ion bombardment of the cathode and a higher cathodevoltage gives a higher electron emission, whereas the maintenance of aDC plasma with hot filaments as the cathode is facilitated by thermionicemission which is not directly related to the cathode voltage.Furthermore, the typical distance between the filament and the top ofthe substrate being coated in a hot filament CVD system is about 0.5-1cm. Hence, the insertion of a grid into this space for uniformdeposition is technically difficult. Any increase of thefilament-substrate separation will decrease the efficiency of hotfilament diamond CVD.

The diamond growth process described in WO92/01828 teaches a filamentrack of resistively heated filaments located between two growthsubstrates such that a DC discharge can be ignited between the filamentsand substrates. Such an arrangement is very similar to the technologydescribed in EP254560. The main difference between this approach and thetechnology disclosed herein is the lack of flexibility in this approachand its inability to process ion extraction and particle bombardmentinduced diamond nucleation and growth. Further, for both thetechnologies disclosed by EP254560 and WO92/01828, the DC plasma currentis directly drawn from the substrate. The problem is that the maximumpower density is limited by the substrate temperature which cannot behigher than the diamond growth temperature range. In the technologydisclosed in the present patent, the DC plasma can be maintained outsidethe filament-substrate region such that the total energy densitydirectly deposited on the substrate surface will not be exceedingly highand yet activated reactants can still be yielded and transported to thesubstrate.

In conclusion, the new diamond growth process disclosed herein providesan enhanced nucleation density and growth rate due to the addition of DCplasmas to the hot filament CVD of diamond with electrode configuration(substrate-hot filament-grid) which allows efficient ion extraction andgrowth precursor transportation towards the substrate during diamondnucleation and growth respectively as compared to thefilament-grid-substrate configuration, and substrate-filament-substrateconfiguration. In addition, the present method provides considerableflexibility in maintaining the DC plasma away from the substrate surfacefor the minimization of excessive substrate heating.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

Therefore what is claimed is:
 1. A method of growing a diamond film byhot filament discharge, comprising: a) positioning a substrate having adeposition surface on a substrate holder in a vapor deposition chamber,providing a grid electrode spaced from said substrate depositionsurface, providing a filament array electrode interposed between saidgrid electrode and said substrate deposition surface; b) flowing a gasmixture comprising hydrogen and gas containing carbon into said vapordeposition chamber and resistively heating the filament array electrodeto a temperature in a range of from about 1800° C. to about 2600° C.,said substrate being heated to a temperature in the range from about600° C. to about 1100° C.; c) nucleating the substrate by biasing thefilament array electrode at a positive voltage with respect to saidsubstrate holder, and biasing said grid electrode at a voltage positivewith respect to the voltage on said filament array electrode to providean effective potential drop between said grid electrode and saidsubstrate for efficient ion extraction toward said substrate; thereafterd) biasing said grid electrode at a voltage positive with respect to thevoltage on said filament array electrode to grow a diamond film on thedeposition surface.
 2. The method according to claim 1 wherein duringnucleating the substrate the substrate is biased at ground potential andthe filament array electrode is biased to a potential in a range fromabout 20 to about 300 Volts with respect to ground, and the gridelectrode is biased to a voltage in a range from about 20 to about 300Volts with respect to the filament array electrode.
 3. The methodaccording to claim 2 wherein during the step of growing the diamond filmafter the step of nucleating the substrate holder and filament arrayelectrode are biased at ground potential and the grid electrode isbiased at a voltage in a range from about 20 to 300 Volts with respectto the filament array electrode.
 4. The method according to claim 2wherein during the step of growing the diamond film after the step ofnucleating the substrate holder is biased at ground potential and thefilament array electrode is biased at a negative voltage with respect tothe substrate holder, said negative voltage being in a range from about−20 to about −300 Volts with respect to the substrate holder.
 5. Themethod according to claim 1 wherein said gas containing carbon isselected from the group consisting of hydrocarbons, hydrocarbonscontaining oxygen and/or nitrogen, hydrocarbons containing halogens,carbon vapor, CO and CO₂.
 6. The method according to claim 5 whereinsaid gas mixture further comprises any one of O₂, F₂, and H₂O and aninert gas, and any combination thereof.
 7. The method according to claim5 wherein said gas mixture is maintained at a pressure in the range offrom about 10 to 500 Torr.
 8. The method according to claim 1 whereinsaid filament array electrode is resistively heated using a power supplyselected from the group consisting of alternating current and directcurrent power supplies.
 9. The method according to claims 1 includingone of heating, not heating and cooling said grid electrode duringgrowth of said diamond film.
 10. The method according to claims 1wherein when said substrate is on said substrate holder said filamentarray electrode is spaced from said deposition surface of said substrateon which said diamond film is being synthesized at a distance less thanor equal to about 2 cm, said filament array electrode being spaced fromsaid grid electrode at a distance less than or equal to about 5 cm. 11.A method of growing a diamond film by hot filament discharge,comprising: a) positioning substrates each having a deposition surfaceon first and second spaced substrate holders in a vapor depositionchamber, providing a grid electrode located between said first andsecond substrate holders and a first filament array electrode interposedbetween said first substrate holder and said grid electrode and a secondfilament array electrode interposed between said second substrate holderand said grid electrode; b) flowing a gas mixture comprising hydrogenand gas containing carbon into said vapor deposition chamber andresistively heating the first and second filament array electrodes to atemperature in a range of from about 1800° C. to about 2600° C., saidsubstrates being heated to a temperature in the range from about 600° C.to about 1100° C.; c) nucleating said substrates on said first andsecond substrate holders by biasing the first filament array electrodeat a positive voltage with respect to said first substrate holder toprovide a plasma having an effective potential drop between said gridelectrode and said first substrate for efficient ion extraction in saidplasma toward said substrates on said first substrate holder, andbiasing said grid electrode at a voltage positive with respect to thevoltage on said second filament array electrode to provide a plasmahaving an effective potential drop between said grid electrode and saidsecond substrate for efficient ion extraction in said plasma toward saidsubstrates on said second substrate holder; thereafter d) biasing saidgrid electrode at a voltage positive with respect to the voltage on saidfirst and second filament array electrodes to grow a diamond film on thedeposition surfaces of the substrates on the first and second substrateholder.
 12. The method according to claim 11 wherein when saidsubstrates are located on said substrate holders, said first filamentarray electrode is spaced from said deposition surface of saidsubstrates on said first substrate holder at a distance less than orequal to about 2 cm, said first filament array electrode being spacedfrom said grid electrode at a distance less than or equal to about 5 cm,and wherein said second filament array electrode is spaced from saiddeposition surface of said substrates on said second substrate holder ata distance less than or equal to about 2 cm, said second filament arrayelectrode being spaced from said grid electrode at a distance less thanor equal to about 5 cm.
 13. The method according to claim 11 whereinsaid first and second filament array electrodes are resistively heatedusing a power supply selected from the group consisting of alternatingcurrent and direct current power supplies.
 14. The method according toclaim 11 including one of heating, not heating and cooling said gridelectrode during growth of said diamond film.
 15. A method of growing adiamond film by hot filament discharge, comprising: a) positioningsubstrates each having a deposition surface on first and second spacedsubstrate holders in a vapor deposition chamber, providing a firstfilament array electrode spaced from said first substrate holder and asecond filament array electrode interposed between said first filamentarray electrode and said second substrate holder; b) flowing a gasmixture comprising hydrogen and gas containing carbon into said vapordeposition chamber and resistively heating the first and second filamentarray electrodes to a temperature in a range of from about 1800° C. toabout 2600° C., said substrates being heated to a temperature in therange from about 600° C. to about 1100° C.; and c) biasing said firstand second filament array electrodes and said first and second substrateholders to produce a plasma to grow a diamond film on the depositionsurface of substrates on the first and second substrate holders.
 16. Themethod according to claim 15 wherein said first and second substrateholders are in opposing relationship, said first filament arrayelectrode is spaced from the surface of the substrate on the firstsubstrate holder at a distance less than or equal to about 2 cm, saidsecond filament array electrode being spaced from the surface of thesubstrate on the second substrate holder at a distance less than orequal to about 2 cm, and said first and second filament array electrodesare spaced from each at a distance of less than or equal to about 5 cm.17. The method according to claim 15 wherein said first and secondfilament array electrodes are resistively heated using a power supplyselected from the group consisting of alternating current and directcurrent power supplies.